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242                               CHEMICAL ENGINEERING
With fine meshed hand screens this is not possible, the eye is the sole guide and the degree of perfection of the test work will depend on the size of the test parcel and the time consumed in shaking the test screen. The theory shows that no matter how prolonged the shaking perfect results cannot be attained. It is better not to attempt to attain it. Where there is much check work to be done with hand test screens the same weight of test samples should be weighed out each time and subjected as nearly as possible to identical manipulations. This is the argument for the employment of a power-actuated battery of shaking screens.1 The latest improvement in this device is an element for shutting off the power after the screens have been shaken any desired time.
Classification of Screens.—Mechanically screens may be divided into two classes: First, those on which the feed advances in a more or less horizontal sheet; and second, those in which a revolving motion is imparted to the material fed.
Under the first type there are the following principal kinds: (a) Stationary, flat and inclined, and flat and inclined shaking screens which includes shaking grizzlies of two kinds; first in which all the bars or punched, cored or drilled sheets move back and forth under the actuating mechanism as an entity; second in the independently moving bar grizzlies the bars move in parallel sets and as one set moves forward the other is moving backward, this gives an excellent transportive effect. (&) Endless belt or bar screens both horizontal and inclined, the first kind being the more common. In a late design of endless grizzly alternate bars are hinged and fall away when the under side of the device is reached, this detail furnishing the means of clearing the bars. When the bars return to their upper position they fall back into place. This type of screen furnishes simple transportive effect free from vibration and complicated mechanical parts but as must be evident it is deficient in means for producing good screening. Pieces of any but the largest sizes are apt on these devices to remain at the point where they are fed until they fall off at the discharge point, (c) Rotary flat screens, the means for advancing the material fed being centrifugal force. The surface of screens of this type is either plane or more or less concaved, (d) Gyratory screens. These have flat surfaces and in the flour-mill type or bolter the gyrations are in a horizontal plane. The Coxe gyrating coal screen is mounted on double cone rollers and has a rocking gyratory motion, (e) Batteries of corrugated cylinders mounted with their axes parallel and in a horizontal plane, have been proposed and used for grading coal or other friable material. The cylinders all rotate the same way and in a direction to advance the material fed. The undcrsizc material falls through the spaces created by the corrugations, which match one another. (/) Revolving vertical cones or stationary cones fed by a revolving horizontal plate at the top.
The second type includes all kinds of revolving screens, reels, trommels, and revolving disc graders of the Rich type which have been employed on wheat, barley, malt, etc. These are usually placed with their long axis horizontal or slightly inclined. Highly inclined cones have also been used for screening of this kind. Reels much used in flour milling and operations of a similar diameter are run at higher Speeds than trommels. Centrifugal reels are run at very high speed, centrifugal force being employed to force the flour or other fine material through the apertures of the bolting cloth. A spiral scraper or brush works the stock through the reel.
Features of Screens of the Flat Type.—If a horizontal flat screen is emplo3red the motion imparted to it must be of a differential character, that is, one giving an accelerated forward motion and a retarded return. This sort of motion enables the grains to pick up the motion on the forward stroke and with the quick return the grains continue to advance past the return point of the complete
1 But there ia no argument for the use of washers riding on the surface of the screen —EDITOR.